1. Field of the Invention
The present invention relates to a squelch circuit for a radio receiver designed to receive communications that occur at random.
2. Description of the Related Art
One typical example of a radio receiver of the above kind is the device which at present forms part of a transmitter/receiver system employing in particular amplitude modulation for radio communication, in particular voice communication, between individuals. Depending on the country, the frequencies used are in the range 26 MHz to 28 MHz, usually known as the "citizen's band".
By their very nature, the times at which communications in this frequency band take place are random so that to receive communications the receiver must be set to a standby mode, after setting it to a particular channel in the given frequency band, so that if a caller comes onto that channel the message is received so that the user can respond if appropriate using the transmitter of his own installation. Consequently, the loudspeaker or other sound transducer connected to the receiver must be connected at all times to the output of the receiver in the standby mode.
Thus when a transmitter/receiver system of the above kind operates in the amplitude modulation mode, unless particular measures are taken, the receiver feeds to the loudspeaker not only a wanted voice signal if any, but also a continuous sound signal produced by the receiver from all forms of interference signals and atmospheric noise reaching the receive antenna. Under these conditions the user constantly hears background noise containing a whole range of audible frequencies (white noise), which is extremely disagreeable after a while.
This is why this kind of receiver has for many years been fitted with a squelch circuit. This device automatically reduces the sound level output by the sound transducer in the absence of a wanted signal and re-establishes this sound level as soon as the received signal is of sufficiently high level.
U.S. Pat. No. 2,770,721 discloses a radio receiver equipped with a squelch circuit controlled by a voltage derived from two voltage components formed in the following fashion.
The output of the detector is connected to a filter which discriminates in the spectrum of the detector a frequency band beginning at an audible frequency of 1000 Hz. The output signal of the filter is amplified and rectified to produce the first voltage component of the squelch control signal.
The prior art radio receiver is described with reference to two versions, one for frequency modulation and the other for amplitude modulation. The frequency modulation version of the receiver includes at least one limiter from which is derived the second voltage component which with the first voltage component forms the resultant squelch control voltage already mentioned.
Moreover, the amplifier connected to the filter for producing the first voltage component includes a device for manually adjusting the squelch threshold to be imposed on the system and from which the squelch circuit operates.
The above document also shows a schematic of an amplitude modulation receiver. In a receiver of this kind there is usually no amplitude limiter. However, in this second version of the receiver described in the document under discussion two amplitude limiters are added (they can usually be dispensed with in an amplitude modulation receiver) so that the second voltage component referred to above is available so that the squelch control signal can be "artificially" generated in the same fashion as described in the context of the frequency modulation receiver.
In both versions of the receiver described in U.S. Pat. No. 2,770,721 the user must adjust the squelch threshold manually employing a potentiometer associated with the amplifier which amplifies the noise signal at the output of the detector.
Apart from the fact that the amplitude modulation version is unnecessarily complex because of the presence of two amplitude limiters, it has a number of other drawbacks. First of all, the noise level in the signal reaching the receiver antenna is not constant and depends on a whole series of highly random factors, for example the time of day (because of movements of the upper atmosphere layers), local reception conditions, especially if the receiver is onboard a vehicle (passing under a bridge or through a tunnel), sunspots, changing propagation conditions, etc.
This is why adjustment of the squelch threshold is useful so that users can do their best to track the changing conditions of the noise level in the receive signal, whence the incorporation into the prior art receiver of the manual adjustment potentiometer previously mentioned. However, apart from the disadvantage of having to adjust the squelch threshold frequently in line with variations in the received noise level (especially in the case of a receiver onboard a vehicle), there is also the risk of the user adjusting the squelch threshold at a time when the received noise level is high, with the result that not only the noise but also all wanted signals below the set level will be eliminated. However, if the received noise level decreases afterwards, the user will not register this because the receiver will remain silent even though wanted transmissions received at a moderate level are no longer buried in noise, the level of which has fallen. Such transmissions could therefore be received normally but they are eliminated because the manual adjustment of the automatic squelch system has not been altered to suit the new, lower noise level.
Another drawback of the prior art receiver (in both the frequency modulation and the amplitude modulation versions) is the fact that the first voltage component of the squelch control signal is formed on the basis of a large part of the energy represented by the wanted voice frequencies, typically from 1000 Hz.
The squelch could therefore become operative unnecessarily if the wanted signal contained a large proportion of these higher frequencies in the voice spectrum transmitted by the radio communication system. This is why the prior art receiver must compensate for this by generating a third voltage component employed in the generation of the squelch control signal, acting in opposition to the first component and based on the lower frequencies in the voice spectrum. This further complicates the system and introduces the risk of distortion of wanted information that the user should normally receive if the latter information is composed entirely or intermittently of frequencies above 1000 Hz.